1. Field
The present disclosure relates to a photo diode, which enables a broadband terahertz continuous wave to have a high output.
2. Description of the Related Art
In a generation of a terahertz (THz) continuous wave by using photo-mixing, a photo mixer serves to convert an incident beating signal into a terahertz continuous wave. Significant variables determining a characteristic of the photo mixer include a fast response rate, large dark-resistance, high carrier mobility, and the like.
Efficiency, a radiation pattern, and the like of the generated terahertz wave have a characteristic of being dependent on an integrated antenna, but in a case where the terahertz wave is applied to a broadband, an absorbing layer of the photo mixer needs to basically have the aforementioned characteristic.
The currently known photo mixer may be generally classified into two elements. One is an element using a low-temperature grown III-V semiconductor having a short carrier lifetime, and includes, for example, low-temperature grown indium-gallium-arsenic (InGaAs), gallium-arsenic (GaAS), indium-gallium-arsenic/indium-aluminum-arsenic (InGaAs/InAlAs) multi-layer, and erbium-arsenic/(indium)gallium-arsenic (ErAs/(In)GaAs). The other is an element using a photo diode structure having a short transit-time, and includes, for example, a unit-travelling carrier photo diode and a pin photo diode. When IR power of several tens of milliwatt (mW) is used in a level of about 10−3 of Terahertz/infrared (THz/IR) power conversion efficiency of the photo mixer using the photo diode structure, the photo mixer has a terahertz output of several tens of microwatt (μW). Compared to the photo mixer using the low-temperature grown III-V semiconductor, the photo mixer using the photo diode structure has a disadvantage in a band width aspect, but an output of the photo mixer using the photo diode structure in a low frequency band is considerably excellent, so that the photo mixer using the photo diode structure is advantageous to be applied to a system having up to about 1.5 THz.
A surface incident pin photo diode among the photo diodes has a characteristic in that responsivity is decreased when a rate of the photo diode is increased by using a thin absorbing layer. In contrast to this, a waveguide photo diode may improve photoelectric conversion efficiency while using a thin absorbing layer, but when light of large intensity is input, a response rate of the waveguide photo diode is decreased by a saturation effect. Due to the aforementioned problems, there is a problem in that it is difficult to satisfy both a broadband and a high output which are the conditions of the photo diode for the application of THz.